This paper reports a simple method using the mesentery as a naturai mountingsubstrate for the study of mesenteric free cells under the scanning electron micros-cope.Mouse mesentery was cut into pieces about Ⅰ square centimeter in size andadhered on to neutral filter paper.The specimen were fixed in 5% glutaraldehyde and1% osmium tetroxide for 20 minutes,each step washed lightly with pH 7.4 PBS,dehydrated with ascending series of alcohol,put into amyl acetate,critical pointdried with CO_2,coated with gold and examined in scanning electron microscope.The morphologic features of free cells in the peritoneal lumen,such as polymorpho-nuclear leucocytes,macrophages and lymphocytes could be observed satisfactorily.We studied in particular the relationship between immunoreactive cells and targetcells(fetal liver cell)injected into the peritoneal cavity,Polymorphonuclear leuco-cytes could be seen invading into and eroding the target cells and macrophage cau-sing their swelling and lysis.Cooperative action of immunoreactive cells and anincrease in number of milk spots were also observed.